There are many technologies on the market used to treat contaminated water for re-introduction into rivers and lakes, irrigation, or human consumption. Each technology has its benefits and downfalls depending on raw water quality, footprint, energy cost, capital cost, end user of the recovered water, and ease of operation. For example, seawater, brackish water, and fresh water have different levels of salinity, which is often expressed by the total dissolved solids (TDS) concentration. Seawater has a TDS concentration of about 35,000 mg/L, and brackish water has a TDS concentration of 1000 to 10,000 mg/L. Another problematic water source requiring treatment is salt brine used to regenerate cationic ion exchange resin, which is typically used for water softening. Such brine has TDS concentrations ranging from 100,000 mg/L, or higher. It is advantageous to recover fresh water from these sources. Water is considered fresh when its TDS concentration is below 500 mg/L, which is the National Secondary Drinking Water standard for the United States.
The two most commonly known technologies for desalinating seawater and other challenging water sources are multi-stage flash (“MSF”) systems and reverse osmosis (“RO”) systems. Both are very energy intensive, and expensive to design, build and operate. MSF systems typically include a large distiller installed next to a power plant, whereby the seawater is used to cool reactors and is heated as a consequence. The latent heat from the power plant drives the distillation process, causing the water to change phase from liquid to gas and then back to liquid through condensation on a cooler surface. The condensed, or distilled, water is almost completely free of contaminants, predominantly salt in the case of seawater. A MSF system can remove up to 99% of the TDS in seawater. The removal efficiency is dependent on the temperature of the incoming seawater and the efficiency of the distilled water collection process. MSF utilizes multiple effect distillation, vapor compression, or both, to help improve how efficiently it can boil, condense and recover the water.
While MSF desalination can achieve high levels of water purity, there are several drawbacks with known implementations. MSF systems incur high capital and construction costs due to exotic material needed for the complex flash chambers and control of corrosion caused by high temperature scaling agents. Operating such systems requires a series of complex processes that require sophisticated controls. The process becomes uneconomical if a free heat source for the incoming raw water, such as cooling water from a power plant, is not available. Finally, a large land area is required per gallon of treated water due to the size of equipment and tankage. A water recovery system that achieves the high purification efficiency of MSF systems without the drawbacks is needed.
RO systems may be used for many water treatment applications, including desalination. RO systems do not require heating of the input water and have a smaller geographical footprint than MSF systems. All RO desalination processes involve three liquid streams: raw input water, low-salinity permeate, and high-salinity concentrate. Most contaminants in the feedwater are removed by forcing the feedwater through a semi-permeable RO membrane with the use of high pressure pumps. Depending on the type of membrane, the feedwater quality, and use of pre-treatment chemicals, the water pressure imparted on the membrane ranges between 250 psi and 1,250 psi for effective removal of TDS. Often, pre-treatment of the feedwater is required before passing through the RO membrane so the membrane does not irreversibly foul, thereby significantly decreasing the removal efficiency of the TDS and increasing operational costs. At its most efficient, RO can achieve 98% TDS reduction for brackish water applications and 75% for seawater applications. While a higher TDS reduction may be accomplished using advanced RO system design, such as cascaded membranes and pre- and post-treatment methods, these augmented designs intensify the drawbacks described below.
There are several drawbacks associated with RO desalination. RO systems require high capital and construction costs due to materials such as membranes, piping, and materials needed to prevent scaling and fouling of the RO membrane. Like MSF systems, RO systems require sophisticated controls to manage multiple processes. Both pre- and post-treatment of the input and output streams is required to achieve optimal efficiency. There is a very high operational cost per gallon of treated water due to the high pressure feed pumps needed to “push” the water through membrane. A less complex, less expensive recovery system that can achieve high water purities is needed.
There are also disposal issues with the concentrate stream, which can be as much as 35% of the volume of the feedwater depending on the recovery efficiency of the RO membrane. Specifically, the RO concentrate stream has very poor water quality due to the combination of feedwater contaminants and pre-treatment chemicals present therein. The typical annual volume of concentrate generated from a 3.5 million gallons-per-day (“Mgd”) inland RO facility used to treat groundwater can range between 84.1 million and 168 million gallons depending on recovery efficiency and demand. This is a very large volume of water with very poor water quality. The concentrate is typically sent to solar drying beds, deep wells, ocean outfalls, or, more often, wastewater treatment plants via the sewer collection system. Unfortunately, there are significant limitations with known means of concentrate disposal. Solar drying beds are very costly due to a large footprint requirement, lack of water recovery, and restricted use only to arid climates. Deep wells and ocean outfalls are falling out of favor due to the adverse ecological and environmental impact. Ocean outfall disposal is becoming more regulated as the density of the concentrate is denser than the ambient seawater. This allows the concentrate to sink to the bottom of the ocean, thereby killing various forms of seagrass and subsequent aquatic life. Conventional wastewater treatment facilities cannot remove the high TDS content in the concentrate without the use of sophisticated tertiary treatment that is uneconomical to design, build, and operate. A large amount of the TDS content ultimately gets passed through the plant to the end user. Because RO systems are frequently used in water treatment facilities, a method and apparatus for treating RO concentrate that removes TDS content as well as other contaminants is needed.
Therefore, it is an object of this invention to provide a method and apparatus to recover clean water from a heavily contaminated input stream, regardless of the contaminants contained therein. It is a further object to utilize multiple effect vacuum distillation to treat the input stream. It is another object that the apparatus be implemented as either a stand-alone system or a component of an existing water treatment facility. It is a further object that the apparatus generates some or all of its own operating power. It is a further object of this invention to destroy organic contaminants that may not be removed by known processes. Another object of the invention is to provide a water recovery apparatus that has low capital and maintenance costs, a small footprint, and reduced ecological impact.